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Freelite assays are the only FLC assays recommended by name in International Myeloma Working Group guidelines, and an involved/uninvolved Freelite sFLC ratio ≥100 is designated as a biomarker of malignancy (Chapter 25.2.1) [167][42]. Guidelines and response criteria that are based on Freelite data should not be applied to other FLC assays because of the poor concordance between the absolute values (Section 8.6.1) [117][215]. A summary of clinical definitions that are based upon quantitative Freelite results are shown in Table 8.9.

Bossuyt et al. [1001] characterised N Latex FLC values in 50 patients (39 κ and 11 λ with an involved/uninvolved Freelite ratio ≥100. When the same cut-off was applied to N Latex FLC assays, only 28% (11/39) κ patients and 48% (11/23) λ patients had an involved/uninvolved N Latex FLC ratio ≥100. Similar findings were reported by Weisel et al. [1070]. After a number of alternative N Latex FLC thresholds were evaluated, Bossuyt et al. [1001] stated that optimal cut-offs were ≥30 and ≤0.03/≤0.02, for κ and λ patients respectively. However, such values would require extensive validation before they can be recommended for routine use.

Popat et al. [215] compared the assessment of response indicated by N Latex and Freelite FLC assays for 42 MM patients who had been treated after relapse. The response criteria used were those defined by Rajkumar et al. [21]. A total of 17/42 patients had measurable disease by both assays and 11 by neither; one patient was evaluable by N Latex FLC only but 13 had measurable disease by Freelite only. For the 17 patients with measurable disease by both assays, there was poor agreement in the assigned response to treatment (Figure 8.12, weighted Kappa 0.75).

Dejoie et al. [923] compared the ability of N Latex and Freelite FLC assays to identify disease progression in LCMM patients according to IMWG guidelines. At clinical progression, Freelite confirmed progressive disease in 15/19 patients, compared to 11/19 by N Latex FLC assays. This study further supports the recommendation that IMWG guidelines based on Freelite data cannot be applied to the monoclonal assays.